The drive to decrease the size of microelectronic and portable devices is putting considerable strain on the performance of dielectric materials currently in use. For high capacity dynamic random access memories (DRAMS) for example, once exotic materials with high dielectric constants such as Barium Strontium Titanate (BST) are being considered for use in microelectronic capacitors (T. Eimori et al. IEDM 93 631 (1993)) For the macroscopic components employed in microwave communications, bulk ceramics such as Ba.sub.2 Ti.sub.9 O.sub.20 and BaLn.sub.2 Ti.sub.5 O.sub.14, with dielectric constants of 40-90 and low dielectric constant temperature coefficients, 0-20 ppm/.degree.C., are currently under active development (T. Negas, G. Yeager, S. Bell and N. Coats, Am. Cer. Soc. Bull. 72 80 (1993) and S. Nishigaki, H. Kato, S. Yano and R. Kamimura, Ceramic Bulletin 66 1405 (1987)).
Low temperature dependence of the dielectric constant is important when high temperature stability of component properties is an issue for a particular application. Such stability is not possible in high dielectric constant materials such as BST due to the high temperature dependence of the dielectric constant, K, associated with ferroelectricity.
Neutralizing the high negative temperature dependence of a ferroelectric with Curie temperature (T.sub.c) below room temperature with the high positive temperature dependence of a ferroelectric with T.sub.c above room temperature is a long standing problem in the field (N. A. Andreeva, O. A. Grushevskaya and V. I. Zhukovskii, Bull Acad. Sci. USSR, Phys. Ser. 24 1281 (1960)). Such a mixture would be expected to have a relatively large dielectric constant, relatively small temperature dependence, and a non-negligible loss factor near room temperature dependent on the quantity of high T.sub.c ferroelectric employed in the balance.
The high temperature ferroelectric perovskites PbNb.sub.2/3 Mg.sub.1/3 O.sub.3 and PbTiO.sub.3 and their mixtures, processing, and chemical substitutions have been extensively studied (e.g. M. T. Lanagan, N. Yang, D. C. Dube and S-J. Jang, J. Am. Ceram. Soc. 72 481 (1989); J. Chen, H. M. Chan, M. P. Harmer, J. Am. Ceram. Soc. 72 593 (1989); M. LeJeune and J. P. Boilot, Am. Ceram. Soc. Bull. 64 679 (1985); and S. L. Swartz, T. R. Shrout, W. A. Schulze and L. E. Cross, J. Am. Ceram. Soc. 67 311 (1984)). The pyrochlore structure Pb.sub.2 (Nb, Mg).sub.2 O.sub.6+x low T.sub.c ferroelectrics have also received some attention (T. R. Shrout and S. L. Swartz, Mat. Res. Bull. 18 663 (1983); and J. D. Siegwarth, W. N. Lawless, and A. J. Morrow, J. Appl. Phys. 47 3789 (1976), but have not been of general practical interest except as impurity phases in ceramics intended to be entirely perovskite phase material (J. Chen and M. P. Harmer, J. Am. Ceram. Soc. 73 68 (1990) and J. Chen, A. Gorton, H. M. Chan and M. P. Harmer, J. Am. Ceram. Soc. 69 303 (1986). A study of Pb.sub.2 (Nb, Zn).sub.2 O.sub.6+x pyrochlore-PbTiO.sub.3 perovskite system found multiple phase ceramics with K near 120 and a TCK (temperature coefficient of the dielectric constant) of approximately 200 ppm/.degree.C. near room temperature (J. Chen, A. Gorton, H. M. Chan and M. P. Harmer, J. Am. Ceram. Soc. 69 303 (1986)). The same authors found the presence of low TCK ceramics in the Bi.sub.2 O.sub.3 --NiO--ZnO--Nb.sub.2 O.sub.5 system (H. C. Ling, M. F. Yah and W. W. Rhodes, J. Mat. Sci. 24 541 (1989) and H. C. Ling, M. F. Yan and W. W. Rhodes, J. Mat. Res. 5 1752 (1990)) with K's in the range 70-100 and corresponding TCK's from +200 to -125 ppm/.degree.C., with lowest TCK's near compositions where K=90.